Recently optical recording media that enable recording, reading and erasing of information have been commercialized and high density rewritable optical recording media that can record qualified animation have been actively developed.
Well-known rewritable optical recording media include phase-change optical recording media which have recording layers either of chalcogenide thin films or semimetal thin films. Chalcogenide thin films comprise Te or Se such as Ge--Sb--Te and In--Se. The semimetal thin films comprise In--Se etc. There are provided well-known magneto-optical recording media having metal thin films such as Fe--Tb--Co for their recording layers. Write once type optical recording media using pigment materials are also known.
In an optical recording-reading device having a phase-change recording medium, recording thin films containing the above-mentioned phase-change materials are instantly irradiated with laser beams focused on submicron-order size optical spots in order to heat the limited portion to a predetermined temperature. When the temperature of the irradiated portion is higher than the crystalline temperature, the crystalline state of the small portions only can be changed, and the state can be changed to the amorphous state if the temperature exceeds its melting point. Once either of the crystalline state or the amorphous state is determined to be the recording state or the erasing state (unrecorded state), recording and erasing of information can be conducted reversibly. Since the crystalline state and amorphous state are distinguishable from each other in their optical characteristics, signals can be read by optically detecting such different characteristics.
In an optical recording-reading device using a magneto-optical recording medium, for example, focused laser beams are irradiated on a magneto-optical recording thin film comprising the above metal thin films, so that the irradiated portions are sectionally heated to a predetermined temperature. A magnetic field is applied to the heated portion in order to control the magnetizing direction of the magneto-optical recording thin film, and thus, information will be recorded or erased.
Uneven tracks are formed concentrically or spirally on the substrate of the optical recording media beforehand, and recording-reading is carried out by using the following device. In this device, optical beams generated from a light source such as a semiconductor laser are focused through a focusing lens to have a small beam diameter and the beam is irradiated on a rotating disk optical recording medium. Recording is conducted by changing the beam's light quantity corresponding to the signals to be recorded. The signals are read by adjusting the quantity of the optical beam to be weak and constant, and by detecting the light reflected from the optical recording medium. Since the uneven track pitches are of a small size on the order of 1 .mu.m, this device is provided with a focusing control, and seek control so that the light spot is irradiated accurately on this track. The focusing control controls the optical beam to maintain an unchanged micro spot diameter. The seek control carries out tracking control so that the optical beam always scans on a track with accuracy, and transfer control to move the optical beam to the desired position of the optical recording medium.
Recording-reading methods for an optical recording medium include a constant linear velocity (CLV) technique and a constant angular velocity (CAV) technique. CLV (including zone CLV) indicates that recording and reading are conducted at a constant linear velocity. CAV (including zone CAV) indicates that recording and reading are conducted at a constant angular velocity. When recording-reading is conducted with a single beam, especially, recording-reading is preferably done with a constant data transfer rate for recording and reading music or image information. CLV is suitable for this purpose. In CLV, recording and reading are conducted at constant linear velocity, so the rotation rate of the optical recording medium is controlled to be high when the optical beam is scanning the inside portions, and the rotation rate is controlled to be low when the optical be am is scanning the periphery. In the zone CLV, the optical recording medium is divided into plural zones in the radial direction. The average linear velocity is controlled to be stable by keeping the rotation rate constant and by changing rotation rates among the zones.
When signals recorded in an optical recording medium are read by using such a device and technique, the power of the optical beam (hereinafter, reading power) should be adjusted to a level where the record signals will not deteriorate due to the optical beam (this phenomenon is hereinafter called read light deterioration). On the other hand, the reading power is required to be as large as possible to increase the output of the signals recorded in the optical recording medium. Therefore, an appropriate reading power value is selected and set considering some elements such as the linear velocity of the recording medium, read light deterioration, and the degree of the signal output.
However, in a conventional optical recording-reading device using CLV technique, the irradiated optical beam may have a larger energy than expected when moving the optical beam on the aimed tracks on the optical recording medium by using the predetermined reading power.
When the optical beam moves from the periphery to the inside of the optical recording medium, the rotation control unit is controlled so that the optical recording medium will have a higher rotation rate in order to maintain a predetermined linear velocity. However, if the stand-up (the rotation reaching the predetermined rate) is delayed, the relative linear velocity between the optical beam and the optical recording medium is lowered. As a result, the energy of the irradiation per time on a predetermined area of the optical recording medium is substantially raised, and the record signals may deteriorate.
An optical beam typically will count the slot-crossing number of the tracking signals and move to a predetermined track, so the optical beam will move while focus-controlling of the optical beam takes place. As a result, the power of the optical beam which is moved and irradiated on the recording track may be stronger than the appropriate reading power while the optical beam moves from the periphery to the inside of the optical recording medium. Such a problem will occur when the rotation rate of the optical recording medium is not raised due to malfunction or failure of the rotation control unit, or when the optical beam scans on the track before it reaches the predetermined linear velocity. As a result, the record signals on the optical recording medium may deteriorate.
In order to prevent this kind of deterioration of record signals, the strength of the motor drive to rotate the optical recording medium can be raised to increase the rotation rate of the optical recording medium to a predetermined value in a short time. However, the rotation rate of recent optical recording media will reach 4800 rpm because of the high transfer rate trend. Therefore, electric power consumption should be increased to obtain a desirable rotation in a short time. Furthermore, strengthening the motor drive is not considered easy for portable information appliances in view of energy-saving considerations.